Electronic controlled relay system



Oct. 10, 1950 G. L. BORELL ELECTRONIC CONTROLLED RELAY SYSTDI V 3 Sheets-Sheet 1 ori inal Filed March 17, 1949 INVENTOR 650565 L. BO/FELL BY FM, 4 6, 7

ATTORNEYS Oct. 10, 1950 G. L. BORELL 2,525,016

ELECTRONIC CONTROLLED RELAY SYSTEM Original Filed March 17, 1949 3 Sheets-Sheet 2 INVENTOR ATTORNEYS $60565 L BOfifLL WW: wvx v QQKMRW 511.

Oct. 10, 1950 s. L. BORELL ELECTRONIC CONTROLLED RELAY SYSTEM 3 Sheets-Sheet 3 Original Filed larch 17, 1949 INVENTOR GEORGE L. BURL-LL ATTORNEYS Patented Oct. 1% 195g s PATENT orricr.

ELECTRONIC CONTROLLED RELAY SYSTEM GeorgeL. Borell, Minneapolis, Minn., assignor to Economics Laboratory, Inc., St. Paul, Minn., a corporation of Delaware Original applicationMarch 17, 1949, sci-m1 No. 81,056. Divided and this application May 3, 1949, Serial No. 91,081

21 Claims. (Cl. 115-320) This invention relates to control and signal apparatus, and especially to that in which a condition or an operation must be controlled or indicated within .close limits. Although the invention has a wide variety of applications, it is particularly adapted to control the concentration of solutions and the like, and is therefore here described in that connection.

Equipment which utilizes cleansing solutions such as detergents, as in dishwashing machines for example, preferably includes apparatus which is intended automatically to maintain the solution concentration at a predetermined value. Apparatus heretofore proposed for this purpose has included a conductivity cell which is immersed in the solution and to which are connected electricalcircuits controlling the feed of detergent to the solution. Such apparatus has not been entirely satisfactory, chiefly because the control system was (1) insufflciently sensitive, (2) slow in response, (3) subject to an inertia effect causing it to overcorrect the given condition and (4) was responsive to an undesirably restricted range of solution concentrations. The apparatus of the present invention overcomes .all of the mentioned disadvantages of the prior systems and yet is simple, readily adjusted, and reliable under severe service conditions.

This application is a division of U. S. patent application Ser. No. 81,856, filed March 17, 1949, of George L. Borell and Marcus 1. Nystuen.

The nature of the invention will be clear from the following description considered in connec-, tion with the drawings, wherein:

Fig. 1 is a circuit diagram of a complete control and signal system including the invention;

Fig. 2 is a circuit diagram of a modification of the apparatus of Fig. 1 which includes provisions for additional controls and signals;

Fig, 3 illustrates an auxiliary signal and control circuit which may be combined with those of any of the other figures to provide further indications and automatic control operations in the event that the prior signals and controls have been ignored or have proved inadequate; and

Fig. 4 is a, circuit diagram of an alternative embodiment of the invention of Fig. 1 employof the present invention, but is included to illustrate a complete application of the invention. This network comprises essentially four arms, two of which comprise the inductance portions L1, In of a power transformer 2, the third arm L: of which comprises the secondary winding of a compensating and impedance matching transformer 3, and the fourth arm of which comprises resistance. of transformer 3 is preferably tapped, or otherwise arranged so that the conductivity cell which is to be connected thereto can be connected at points of different inductance values for the purpose later to be described. To this end a switch 5 is arranged to contact-points 6, l and 8 on primary 4. Connecting means are provided to connect to the selected portion of primary 4 a conductivity'cell 9 which fundamentally may I comprise two electrodes insulated from each other and arranged to be immersed inthe solut'ion, the concentration of which it is desired to control. It is preferable; in accordance with the invention, that this cell include temperature compensating means so that change in temperature of the solution will not appreciably affect the balance of the bridge, since, in the application of the invention her'e described by way of example, it is the solution concentration rather than the temperature which is intended to be controlled. A novel form of conductivity cell, especially adapted for use in connection with such a system is described and claimed in coending application Ser. No. 71,418 of George L.

Borell and Marcus I. Nystuen, filed January 18, 1949.

It will be evident that the fundamental system of the invention is adaptable to the control into the bridge network at the L3 arm would be of appropriate type and design.

The fourth arm of the bridge network includes resistors R1 and R2, of which resistor R1 in the present embodiment may comprise the temperature compensating resistor above referred to which is included in the conductivity cell. Resistor R: may conveniently comprise an additional variable element. resistors R1 and R2 are not here suggested because they would ordinarily depend on the electrical characteristics of the cell 9, or some other sensing device or element.

Across one pair of conjugate points of the bridge network is connected the primary It The primary 4 The actual values of 3 of an output coupling transformer Ii, having a secondary winding I2. One of the conjugate points to which primary II is eiiectively connected includes a potentiometer it, the resistor element of which is connected across an appropriate number of central turns of the secondary inductance Ll-L2- The resistance of potentiometer i6 may conveniently be of a value or 50 ohms and be connected across a portion of the mentioned secondary winding which represents about 6 volts. This potentiometer constitutes a titration control, the purpose of which will be later explained. The other pair of conjugate points effectively comprise the extreme terminals of the inductances L1 and La by means of which power is impressed on the bridge network. For this purpose transformer 2 has a primary winding l3 which, in turn, is connectible through power switch H and fuse II to a power line here assumed to be or about 120 volts, 60 cycles alternating current.

The secondary winding l2 of bridge output transformer ii is connected to the control grid and cathode, respectively, of a suitable amplifying tube H, such as type 6V6, for example. The primary and secondary windings [0, I2 of transformer II should have impedance values appropriate to couple the indicated circuit elements. In the present instance, a ratio of about 1:10 is satisfactory. This transformer also conductively isolates the bridge network from the grid of tube i'I, which is necessary because that network is necessarily grounded through the conductivelycell electrodes and the solution in the wash tank.

Theanode circuit of tube i1 includes the windings of two relays I8 and i9, respectively. These relays may be of any suitable type and for the present purposes may be substantially alike in their characteristics and construction. The relay coils may be of, say, 2500 ohms each. Relay i8 may be adjusted to be actuated by a current of about milliamperes, and relay i9 may be actuated by a current of slightly'more than that, as will be more clear from the description of the operation of the system. A condenser is shunted .across the coil of relay ID to prevent it from chattering when used with alternating cur rent or with pulsating direct current, which results from rectifying action of tube II. This condenser should be of large capacity, such as 40 microiarads or more, but might be omitted ii the anode circuit, were operated on direct current, which is entirely feasible although usually not so convenient. Contacts 2i and 28 of relay it when closed energizessolenoid valve 22, or any other other desired component or mechanism which it is desired to control. Resistor 40, connected in the cathode return lead of tube H, which in this case may be of about 300 ohms,

.should be such that the relay l8 will be positively deactuated in the balanced condition of bridge network I. By means of this resistor, amplifler tube i1 will operate on the steepest part of its transconductance curve. Resistor 40 also tends to minimize variations in the anode current of amplifier tube I! which result from load variations in the anode circuit.

The solenoid valve, here shown to be connected to the control circuit by suitable means, may be assumed to control the supply of detergent to the washing tank in the washing machine so that when the valve is open detergent will flow through conduit 66 from source 81 and when it is closed the flow will cease. If the apparatus of the invention were applied to another use this valve might control the iiow of any iiuid desired, or it might be replaced by any desired controlling or signalling or other device. In the same circuit with the solenoid valve is connected a signal lamp 22 which will be lighted whenever the solenoid of this valve is energized. Also connected in this same control circuit by the closing of contacts 2i is the cathode heater 24 of diode 2|. This tube, may, for example, comprise a rectifier tube of type 11723, although any equivalent tube, delayed-action relay, or other suitable device may be substituted. The cathode and anode of diode 2| are connected so as to form a'controlled bleeder, or auxiliary, circuit which shunts the coil of relay I! through resistors 20 and 21 one of which, here resistor 21, should be variable. These two resistors may be combined into a single unit if preferred.

The mentioned controlled bleeder circuit, which is further discussed below, is an important feature of this invention inasmuch as-it makes it 22 and 22 energized by the closing of contacts 2| and 28.

The second relay, II, is connected, as shown, in series with relay II in the anode circuit'of tube i1, viz., between the anode '30 of tube 11 and the ungrounded side of the power line. This relay I 9 which is assumed to be adjusted so as to be actuated after the actuation of relay II in response to increase of anode current, has shunted across its winding another condenser 21 which may be similar to condenser 20. The purpose of condenser. is to effect the automatic pulsing of this relay so that the pulse rate will be increased with increase of current flowing through the relay winding, until that current reaches a value which is sufllcie nt to hold the relay steading to discharge it at a predetermined rate depending upon the value of the resistance. Closure of contacts ii of relay is energizes a supplementary circuit which may be employed for any desired purpose. Here closure of these contacts actuates a warning buzzer 25 and a signal lamp or lamps 36. Signals 25 and 36 may conveniently be operated on the 6.3-volt line which is energized by the secondary winding 31 of transformer 2, used to energize the cathode heater of amplifier tube l1. Signal lamp 3! will be li hted as long as power switch II is closed, to indicate that the entire system is connected for operation.

Adjustment and operation of Fig. 1

The general applicability of the invention and its many advantages will be better understood from the following description of the operation of the system of Fig. 1. For the purpose of this description it may be assumed that the apparatus of the invention is employed to control the concentration of the detergent solution in the wash tank of a dish-washing machine. The conaiieuvity cen 0, having other types.

s beenv preferably secured in a wall of the wash tank or located elsewhere ills actuated contacts 22 immediately close.

in the liquid system so as to be suitably immersed in the washing solution, is connected in the bridge network of Fig. l as shown.

Under various conditions it is sometimes required to employ washing solutions of considerably diilerent compositions andconcen trations. As a result, the actual conductivity of such solutions will vary considerably, and the resistance and capacitive effects of any given conductivity cell will vary accordingly. To accommodate this range of operating conditions, the compensating transformer 3 may be used in themanner explained in copending application Ser. No. 81.856. The slider on potentiometer P0, which for convenience may be called a titration control," ad- Justs the eflective balance point of'the bridge. Consequently, adjustment of control Ii determines within very close limits the degree of concentration of the solution which is automatically maintained by the system of the invention.

It being assumed that the apparatus is ener- 'gized by closure of power switch it and that the bridge network has been balanced with the conductivity cell immersed in the solution oi desired concentration, the anode current of tube II will then be too small to actuate either of relays l8 and I9. When the concentration of the solution decreases, as occurs in normal operation of the machine, the balance of the bridge network will be destroyed to a degree proportional to the dilution of the solution. When the anode current is increased sufllciently to actuate relay I 0, contacts 2| and 28 will immediately close. The current at which the relay is actuated and, therefore, the concentration of the diluted solution at which it is desired that more detergent should be added can be adjusted by changing the air gap between the-armature and the pole-pieces of relay i8, or by changing the strength of the armature spring.

Actuation of relay it, as above described, actu-,

ates solenoid valve 22 controlling the feed of detergent into the wash tank. If the rate of feed is insufllcient to restore the concentration to the predetermined value such as would be the case if the detergent supply were exhausted, or if an unusually large amount of rinse water should flow into the wash tank, the degree of concentration will further decrease with a resulting further increase of anode current through the relay windings. When the anode current thus increased, reaches a predetermined value,

relay 19 will also beactuated to close its contacts 32 and 34. Closure of contacts 24 actuates waming signal devices 35 and I0, and any other apparatus which may be connected to terminals 65. If desired, another solenoid valve may be connected to terminals 65, either directly or through a third relay" and this valve employed to feed additional detergent into the wash tank in order to hasten the required increase in concentration. Such apparatus is illustrated in Fig. 4.

The circuit of relay is is arranged to be selfcycling, viz., to produce intermittent operation, which has been found to be preferable in systems of the type herein described, as well as in many This pulsing action is eifected by the parallel connection of condenser 2| and the winding of relay l9 which are shunted by bleeder resistor 33 upon closure of contacts 22. When current flows through the winding of relay I! a charge is built up on condenser 8|. When relay This would immediately deactuate the relay were it not for the charge on condenser I I which maintains the current through relay is until it .is bled off through resistor 33. The rate of discharge and hence the basic pulse .rateor self-cycling of the relay, is determined by the magnitudes of the resistor 23 and of the condenser 3|. As a result situation 'and make possible the use of a given sensing device and bridge network over a wide of this arrangement it will be seen that the actual pulse rate will vary with the charge on condenser ll which in turn is determined by the current in the anode circuit of tube ii. Consequently, the pulse rate of relay I! will increase with decrease of solution concentration, so that as the solution becomes weaker the signal devices 35 and 36 will be actuated with increased rapidity, the eifect of which is to make the warning signal become more urgent. ,When the solution concentration becomes very weak, the unbalance of' the bridge network will be so great that relay 1! will cease pulsing and will hold up continuously until the conditions are corrected. -In the example given,

rela-y I9 will begin to pulse when the solution is, say, 10% below normal concentration, but this point'of operation can be adjusted conveniently by adjusting the strength of the armature spring .on the relay l9, or by adjusting the air gap between the armature and pole-piece of the relay.

An additional feature of the invention, which is of considerable importance and to which are due many of the advantages above set forth, re-

sults from the elements which compensate (or I over-compensate) the so-called operating diil'erential of relay Hi. It is well known that all r lays of the commercial type have an inherent ope ating diilferential, sometimescalled marginal operation, by which is meant that the relay actuates at a current higher than that at which it deactuates. This differential is usually about 20 percent. For example, a relay which actuates at, say, 10 milliamperes, will not release or dea'ctuate until the current decreases to, say, 8 milliamperes. As a consequence. a control system which includes such a relay cannot be accurate within close limits because, while the initial control action may be substantially instantaneous the relay differential will cause a considerable lagin the deactuation. This defect is present in most control systems; and even in those in which an attempt has been made to compensate the diilerential action, the improvement has been only partial.

This feature of the present invention, is, therefore, not only important to the operation of the apparatus here described, but likewise has valuable application in many other-control systems.

As a result oi the invention, the operation of the main control relay is is stable and positive, and at the same time occurs at exactly the required instants or phases in the control cycle. To this end, closure of contacts 2i and 28 not only actuates the required mechanism such as solenoid valve 22, but also energizes the cathode heater 24 of diode 25. When diode 25 begins to conduct, it connects resistors 26 and 21 across the coil of relay l8. These-resistors may be of 6800 and 5000 ohms, respectively. If the effective resistance of this shurit circuit is such as to pass about 2 milliamperes, for example, the reduction of current through the relay will cancel th mentioned two-milliampere operating difl'erential. The result is that the relay will drop out even though the anode current remains at 10 milliamperes instead of delaying until that current decreases to 8 milliamperes which would be the case in the absence of the mentioned shunt cir- 7 cuit. It the shunt circuit is adiusted'to pass, say, 3 milliamperes, the relay will drop out even though the anode current might increase to 11 milliamperes. This over-compensation or the differential is usually preferred-because it provides not only a more positive and sensitive relay action but'also an anticipatory control which is of great value. When the relay thus becomes deactuated the heating current to tube 25 is cut oil. When the cathode of diode25 cools to a point where the tube is no longer conductive, no bleeding occurs and the original condition prevails unless the sensing circuit (bridge network) has been brought back to the balanced point and the anode curv rent of amplifier tube I! has been thereby reduced an appreciable amount.

A second important feature of the controlled bleeder circuit just discussed results from the self-cycling operation thereof, which eliminates any tendency of the control system to overshoot, or to over-correct the condition in response to which the system operates. First, by adjusting sensitivity control resistor 21 (reducing the resistance) the operating diflerentia1 may be reduced electrically to a value less than zero (viz., it is over-compensated) during each operating cycle as just described, so that it is possible for the system to detect any decline in solution concentration, then to cause the feed valve to be opened for a short time, such as to seconds, to increase the concentration, and finally, before the system has recovered entirely, the control, by virtue of the self-cycling feature, causes relay 18 to drop out and to close the feed valve. In other words, by virtue of the self-cycling control, the relay is de-energized before it receives a signal from the conductivity cell that the condition has been fully satisfied. The reason that it is important that the control should anticipate the satisfied condition, is that it requires an unavoidable time period for the added detergent to become mixed with the solution in the wash tank and then to influence the conductivity cell. However, this time period will vary under different conditions, for instance, depending on the size of the tank, the time required to dissolve the detergent or other chemicals added to the solution and the facilities for mixing them. The same or corresponding situations prevai1 in many other .control systems, such as in connection with heating, refrigerating and ventilating systems, where there are various delays before the detecting device is sufliciently influenced by the required change in conditions, which usually results in over-correction. In accordance with the invention the magnitude of the required anticipatory factor can readily be selected so as to provide the correct value to compensate exactly. for the lag in any particular system.

The desired delay in the self-cycling operation of the controlled bleeder circuit is here attained by the use of a diode 25 of a type which inherently heats slowly and which heating is further delayed by means of a series resistor 38 connected in the heating circuit. In this instance this resistor may be of 1,000 ohms. As a result, tube 25 does not begin to conduct until about 10 seconds have elapsed. If resistor 39 is adjustable, the length of this delay may be preselected. Meanwhile, the detergent has been flowing into the wash tank. Under normal conditions the relay will drop out again within a period or 10 to 20 seconds due to decrease in relay current caused by bleeding action through tube 25 which is now conductive.

\ valve 22 of Fig. l, and that the solutionconcen- During the next 30 to 50 seconds after the relay drops out, the cathode of diode 2i slowly cools because its heater circuit is no longer energized.

As a result, the amount or current shunted from relay 10 through diode 28 gradually decreases' to. zero, thus allowing condenser 20 to recharge and current through coil of relay ll to reach the threshold value, at which time the system is peaked" again ready for the next cycle.

During this 30 to 50 seconds waiting period, the new detergent material will have had sumcient time to become thoroughly dissolved and mixed, into the wash solution. The complete cycle as above described normally requires about one minute with the apparatus here specifically mentioned.

If the amount of detergent thus added is sufficient to bring the solution back to the proper concentration, the feeding relay l8 will remain open and no further addition will be made until the detergent concentration is again decreased. If, however, the amount of detergent added in this short feeding period has been insuflicient to bring the solution up to the proper strength the feeding relay I0 will close as soon as it is "peaked" and detergent will again be added for a short period of time. This cycling will continue as long as dilution occurs, but the ratio of "on to '00?" intervals will change automatically depending on the degree of dilution.

The manner of operation by which the menalso assuming, as above mentioned, that the rev sistance of the actuating coil of relay i8 is about 2,500 ohms, the bleeder circuit shunted across that coil may vary from infinity when the oathode is cold to about 6,000 or 8,000 ohms when the cathode is heated to its normal operating temperature. Inasmuch as the conductance of this bleeder circuit is dependent upon the cathode temperature, the change in the bleeder current during ,each control cycle follows roughly the asmoptotic curve of a thermocycle. Consequently,-the bleeder current approaches maximum value quite slowly and similarly approaches zero value slowly. Under these circumstances, the exact time when relay IE will close or open again will be governed by the relationship oi the magnitude of the total anode current 01' tube I! to the peak values (high and low) of the bleeder circuit current, and the time intervals tration is at a value above normal operating level for which the control system as a whole has been adjusted, the result will be that the anode current of tube II will be reduced to a point at which relay I8 will no longer close, or pull in, even though the resistance of the bleeder circuit approaches infinity. This condition'ot the bleeder again drop out as soon as the bleeder circuit shunts oi! the required current (here 2 milreduced to a value slightly less than the mentioned normal operating level, the anode current of tube I! .will be appreciably increased. as a result or which -a longer time. perhaps to seconds, will be required for the cathode of the diode to reach a temperature high enough to permit the diode to conduct 2 milliamperes plus the equivalent of the increase of anode current of tube I! (which may amount to, say, 3 milliamperes in all) 'in order to permit the relay again to drop out, or become deactuated. From this it is evident that even though the diode cathode temperature reached a much higher value in the latter instance, it will cool initially at a relatively high rate and rapidly reach a temperature at which the current shunted from relay ll is reduced to a value permitting the relay again to pull in, even-though the diode be still conducting a small amount of bleeder current. In the described example this latter period might comprise 40 to 50 seconds. The duration of the heating cycle of the subsequent operation will remain substantially the same in spite of the fact that under the described conditions the heater 0! the diode would not be entirely cold at the =beginning oi such subsequent cycle.

As the solution concentration is still further reduced, the "time on" will be further increased and the time off" further decreased, until a value of concentration is reached at which the maximum bleeding current (in this case assumed to be 3 milliamperes) is insufficient to bypass the anode current to a value below that which will hold relay is closed. This condition may be termed the lock-in" condition. In one commercial embodiment of the invention the variation oi time on" to time oil" ranged from 20 seconds on to 30 seconds "01!" at a solution concentration Just above that eflecting "lock in," and at the other extreme the variation ranged from 10 seconds -on" to 50 seconds "oil!" at a concentration just below that effecting "lock-out." Thus the system of the present invention provides a control which anticipates the demand and therefoke anticipates the normal control point.

Although the type 1l7Z3 diode has proved to be an economical and eilective device by which the mentioned bleeder and delay operations are achieved, the, invention is not limited to .such device because it may be replaced by other types oi delayed-action relays and switching devices, such as thermal relays and other apparatus known in the art.

The modification illustrated in the circuit diagram oi Fig. 2 is basically similar to that of Fig. 1, and corresponding circuit elements in the two figures are similarly numbered. A bridge network or other source of control signals may be connected through transformer II. The apparatus of this arrangement diii'ers from that of Fig.

l chiefly in that it includes a third and auxiliary relay Ii having contacts 42 and 3. Relay ll may be or a type similar to that oi the other relays and, as before, the winding thereof is shunted by a condenser 44 which may be similar to condensers 20 and II By adjusting the spring or the air gap oi this relay it may be caused to be actuated when the input signal becomes greater than is required to actuate relay ll. Under these circumstances if an auxiliary solenoid valve ll be connected so as to be energized by closure oi contacts 42, 0, an auxiliary source 88 or detergent may be fed to the wash tank. Indicating lamp l0 connected in the circuit of solenoid 45 will indicate operation or solenoid valve 48 or any other equipment which might be substituted for or added to valve 45. I! desired. additional auxiliary relays and equipment may be similarly connected to be operated simultaneously or in succession as may be required. Furthermore, and as explained in connection with Fig. 1. additional indicating or other elements may be connected to terminals ll under the control of relay ll.

Operation of Fig. 2

In the illustrated example, wherein it is assumed that the system or Fig. 2 is to be used for the control of solution concentration in a dishwashing machine, the operation is briefly as rollows:

when the' bridge network coupled through transformer II is balanced, viz., no signal potential is impressed on tube II, the control apnaratiu is in standby position, but when the bridge becomes slightly unbalanced due to dilution of the solution, a signal potential is impressed on the grid of tube I1, and increased current flows in the anode circuit of that tube and thus through windings of relays i8, ll and Is which are connected in series in the anode circuit. Due to the relative adiustments of the relays, relay I Q will operate first to add detergent to the wash tank. This relay, being connected in aself-cycling cirouit also arranged to compensate for the relay diii'erential, as previously explained, will provide more unbalanced and the anode current oi tube I! further increased, which will cause the actuation of relay ll. Operation of this second reay will increase the feed rate of detergent, and if the conditions are satisfied, relay I will open before relay I! because it'is adiusted to operate on a higher current value. cycling and differential-compensating reflne- .ments are not required in the circuit of relay 4 I, but could be provided if desired. In the event that the actuation of relay I still rails to meet the demand as called for by the unba anced condition of the bridge network as wou d be the case it both detergent suppies were exhausted, relay II will be actuated and, as in the case of Fig. 1, will give warning -signals audible from buzzer 35 and visible from lamp It, or otherwise, as previously suggested.

Figure 3 The auxiliary control circuit illustrated in Fig. 3 is adapted to be connected to the terminals 88 associated with relay IQ of Fig. 1. This auxiliary circuit includes a relay 4! which may be of the time-delay type, such as a thermal or dash-pot relay. To the output or contact circuit of this relay, additional signalling devices such as buzzer Ordinarily the self- 3. Control apparatus including a vacuum inherent operating diii'eren'tial 01' a certain current value and including. an actuating coil connected in said anode circuit, an electrically operated device, connections from said relay to said device for operatingthe same, a compensating circuit connectible in shunt to said actuatin coil, diilerential-compensating impedance means connectible in said compensating circuit and being 01' a magnitude which 'passes current of at least said certain value, and means additional to said signal potential operative autohaving a grid electrode adapted to be coupled to a source of control signal potential, and having an anode circuit, a relay oi the type having an M sai;

through said coil below the relay actuation valu thereof.

5. Apparatus according to claim 3' which in- I cludes a second relay having an actuating coil connected in series with the coil of the firs mentioned relay, a condenser connected in parallel with the coil 01; said second relay, and a bleeder resistance connectible across the coil of said second relay and said condenser by opera tion of said second relay, the resistance value of said bleeder resistance and the capacitance value of said condenser being such that said second relay is automatically actuated and deactuated ata pulse rate which increases with increase of current in said anode circuit, and a responsive device connected for actuation by said second relay.

6. Apparatus according to claim 5 which also includes a third relay having an actuating coil, said third coil being connected in series with the first and second relay coils, said third relay being adjusted to be actuated by-more current than is required for actuation oi the first relay and less current than is required for actuation of the second relay, and a second electrically actuated device connected to be operated in response to actuation of said third relay.

'7. Apparatus according to claim 3 character- 4 ized in that said diiIerential-compensating impedance is of a magnitude which passes current greater than said certain value so as to overcompensate said relay differential whereby said relay deactuates at a current value higher than that at which it acutates.

8. Apparatus according to claim 3 characterized in that said impedance comprises a resistance, and said means automatically connecting and disconnecting the resistance intermittently includes a diode having a cathode heater connected to be energized by actuation of said relav, said resistance being connected in series with the anode and cathode of said diode across the actuating coil of said relay, and a condenser is connected in parallel with said coil.

9. Apparatus according to claim 3 characterized in that said impedance comprises a resistance, and said means automatically connecting and disconnecting the resistance intermittently includes delayed-action relay means operative in response to and a predetermined time after actuation of said relay to connect said resistance in shunt to said relay coil, the magnitude of said resistance being such as to reduce the current '10. Apparatus accordingto'claim 3 which includes a second relayhavingan actuating coil connected in series with the poll or the first-mentioned relay, a condenser connected in parallel with the coil of said second relay and a bleeder resistance connectible across said condenser by operation 01' said second relay, the resistance actuated at a pulse rate which increases with increase 01' current in said anode circuit, and a signal device connected for actuation by said second relay, control mechanism and a motor for driving the same, a third relay connected to be actuated by said; second relay, contacts on said third relayf arranged to open upon actuation or. said third relay and connections from said contacts to control the operation of said motor.

11. Apparatus according to claim 8 in which said means automatically opening and closing said shunt circuit comprises a-switch connected to said circuit, a cam actuating said switch, and means operating said cam independently of said relay.

12. Apparatus according to claim 8 which includes a second relay having an actuating coil connected in series with the coil of the first-mentioned relay. a condenser connected in parallel with the coil oi said second relay, a cam and a switch havingcontacts operated by said cam, a bleeder circuit including in series said switch contacts, contacts of said second relay, a bleeder resistor and said condenser, whereby closure of said switch contacts connects said bleeder rethe same, difierential-compensating impedance of a magnitude which passes current of at least I 14. Control apparatus including a vacuum tube having a grid electrode adapted to be coupled to a source of control signal potential, and having an anode circuit, a relay having an actuating coil connected in said anode circuit and being of a type which operates in response to current 01' a certain value, a compensating circuit including impedance elements and said actuating coil,

' certain of said elements being connected in said circuit by operation of said relay and being of impedance magnitude such that said relay is au- A :tomatically actuated and deactuated in cycles, the period of actuation in relation to the period oi deactuation in each cycle being thereby a function of the current in said output circuit.

16. Control apparatus including an electrondischarge tube, means for impressing a control signal potential on said tube to'control the operation thereof, said tube having an output circuit, a relay connected in said output circuit so as to be actuated by current therein, a thermionic discharge device having a cathode and an anode circuit, said device being of the type of which the effective resistance of the anode circuit varies with th heating of the cathode, heating means connected to heat said cathode, connections between contacts of said relay and said heating means such that said heating means is energized and deenergized concurrently with actuation and deactuation of said relay, said relay being connected in shunt to said anode circuit such that the operation of said relay is in part determined by the effective resistance of said anode circuit, and responsive means connected to be operated by said relay.

17. Control apparatus including anelectrondischarge tube, means adapted to impress a control signal potential on said tube to control the operation thereof, said tube having an output circuit, a relay connected in said output circuit so as to be actuated by current therein, an auxiliary circuit including impedance, a delayed action switching device connected to said auxiliary and anode circuits so as effectively to increase and decrease impedance in said anode circuit and thus vary the actuating current to said relay, connections between said relay and said switching device such that said device is energized and deenergized concurrently with operation of said relay, and responsive means connected to be operated by said relay.

18. Apparatus according to claim 17 in which said delayed action switching device is of the type which, when energized, slowly varies the impedance in said auxiliary circuit from a first to a second value and, when deenergized, slowly varies said impedance from said second to said first value.

19. In apparatus according to claim 1'7, means for varying the magnitude of current in the output circuit of said tube from a minimum value to a maximum value in response to the degree of control signal potential, said relay being held in 55 lock-imposition by a certain minimum value of current between said first-mentioned minimum and maximum values, and the impedance of said auxiliary circuit being such as effectively to decrease the current to saidrelay to a value no less than said certain value even when the current in said output circuit is atsaid maximum value.

20. Control apparatus including an electrondischarge tube, means adapted to impress a control signal potential on said tube to control the operation thereof, said tube having an output circuit, a relay connected in said output circuit so as to be actuated by current therein in response to said signal potential, circuit means connected to said relay for automatically causing intermittent actuation and deactuation thereof independently of change of signal potential, and means including an auxiliary circuit effectively established by operation of said relay which varies the ratio of actuation to deactuation periods of said relay in accordance with changes in said signal potential.

21. Control apparatus including a vacuum tube having a grid electrode adapted to be coupled to a sourceof control signal potential, and having an anode circuit, a relay of the type having an inherent operating differential of a. certain current value and including an actuating coil connected in said anode circuit. an electrically operated device, connections from said relay to said device for operating the same, a compensating circuit connectible in shunt to said actuating coil, diflerential-compensating impedance means connectible in said compensating circuit and being of a magnitude which passes current of at least said certain value, a delayed action switching device effectively connected in said circuit and operative to close the same a predetermined period after actuation of said relay and to effectively open said circuit a predetermined period after deactuation of said relay, and adjustable means associated with said device for predetermining said periods.

GEORGE L. BORELL.

aarnanncas crran The following references are of record in the file of this patent:

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